<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <script type="text/javascript">
        var testUndefined = function () {
          var obj = {}
          var undefined = 'underscore'
          var window = {
            'undefined': 'qianlongo'
          }
          console.log(window) // {'undefined': 'qianlongo'}
          console.log(undefined) // underscore
          console.log(window.undefined) // qianlongo
          console.log(obj.name === undefined) // false
          console.log(obj.name === window.undefined) // false
          console.log(obj.name === (void 0)) // true
        }
        testUndefined()
    </script>
</body>
</html>